A turbo-supercharger driven by a turbine in the path of engine exhaust is known to improve the output of an internal combustion engine.
It is also known that the efficiency of the compressor of such a turbo-supercharger can be indicated by parameters comprising the pressure ratio of the top and bottom flow of the compressor, and the flow rate of the gas passing the compressor. When the pressure ratio deviates from the appropriate pressure ratio range (normal operation range) corresponding to the gas flow rate, and enters an abnormal operation range, undesirable phenomena, such as surging, choking or over-speeding of the turbine, are experienced, whereby the operating condition of the compressor should be established so as not to enter the abnormal operation range.
For example, when sudden deceleration is effected by releasing the accelerator pedal during high-speed steady driving, the fuel injection rate is decreased, thereby causing a rapid reduction in the exhaust flow rate. During this time, the operating condition of the compressor sometimes enters the abnormal operation range, thereby generating surging.
In addition, in order to improve exhaust performance, an intake air control means or mechanism such as a catalyzer is provided in an exhaust path, and an inlet throttle means or mechanism is provided in the inlet path, so that the air excess coefficient of the intake air introduced to the engine is controlled according to the driving conditions. Or, it is known to provide an EGR (Exhaust Gas Recirculation) device for introducing only part of the exhaust into the inlet path according to the driving conditions, a pipe interconnecting the exhaust path and an inlet path (EGR pipe), and an EGR valve controlling the flow rate of the EGR pipe.
For example, a post-exhaust process making use of, for example, an NOx trap catalyzer and a diesel particulate filter (DPF) should effect so-called regeneration control in which the excess air coefficient is controlled to be rich during operation in order to regenerate the purification function. At the completion of such regeneration control, the fuel injection rate (that was increased in order to control the excess air coefficient to be on the rich side) is decreased, so that the exhaust pressure decreases. Due to this change in the exhaust pressure, surging may be generated at the turbine compressor.